Golf balls

Abstract: In a three-piece solid golf ball comprising a center core, an intermediate layer, and a cover, the center core (1) has a diameter of at least 29 mm and a specific gravity of less than 1.4, the intermediate layer (2) has a thickness of at least 1 mm, a specific gravity of less than 1.2, and a hardness of at least 85 on JIS C scale, and the cover (3) has a thickness of 1-3 mm. The ball has a good total balance of properties in that feeling and controllability are improved at no sacrifice of flying performance and durability.

Description: This specification relates to golf balls. Among a variety of golf balls, thread-wound golf balls and solid golf balls are now popular. The solid golf balls are currently increasing to mainstream popularity. Among them, two-piece solid golf balls consisting of a core and a cover are the most widespread. Most amateur golfers are fond of two-piece solid golf balls, which have excellent flying performance and durability despite the disadvantages of a very hard feel on hitting and low control due to rapid ball separation on hitting. However many professional golfers and skilled amateur golfers who stress feeling and control prefer wound golf balls, especially wound golf balls using a soft balata cover, to two-piece solid golf balls. The wound golf balls are superior in feeling and control, but inferior in flying distance and durability to the two-piece solid golf balls. Seeing that two-piece solid golf balls and wound golf balls have contradictory characteristics as mentioned above, players make a choice of ball depending on their own skill and taste. In order to develop solid golf balls having a hitting feel approximate to the wound golf balls, two-piece solid golf balls of soft type have been considered. For such two-piece solid golf balls of soft type, soft cores must be used. If the cores are soft, however, repulsion becomes low with a concomitant loss of flying performance; Furthermore durability is considerably deteriorated. That is, the superior flying performance and durability which are characteristic of two-piece solid golf balls are lost and, in an extreme case, the balls become unacceptable for practical use. Controllability, which is desirable even on full shots with drivers, is most important on control shots like approach shots. If for example a shot needs to fly beyond the bunker but then only a short distance from the green edge to the cup, a player who is either professional or amateur will naturally wish to hit a ball with a minimal run. Such controllability of a golf ball largely depends on spin properties. On a full shot with a club having a relatively large loft, the club loft is more dominant than the ball nature so that almost all balls are given an appropriate amount of spin and few balls overrun. However, on a approach shot over a short distance of 30 or 50 yards, balls will significantly vary in run or controllability. The major factor causing such differences is not the basic structure but rather the identity of the cover material. In two-piece solid golf balls, however, covers made of soft material are effective for improving controllability but detrimental for gaining flying distance. FR-A-2666018 (equivalent to GB-A-2248067 and US-A-5528871) discloses a three-piece solid golf ball comprising a centre core, an intermediate layer around the core and a cover enclosing both. The intermediate layer is preferably a mixture of an ether block copolymer and one or more ionomers, and has a Shore D hardness in the range from 25 to 50 (corresponding roughly to 40 to 75 on the JIS C scale), while the outer cover is harder than the intermediate layer and typically of Shore D hardness in the range from 40 to 55 (about 64 to 80 on the JIS C scale). An aim herein is to provide a novel and useful solid golf ball construction. A preferred aim is to achieve a good feel and controllability while maintaining the good flying performance and durability which are characteristic of solid golf balls. In connection with a solid golf ball having a core forming the center and a cover forming the outermost layer, we have found that by providing a relatively hard intermediate layer between the center core and the cover, the center core can be made relatively soft so as to improve feeling and controllability without deteriorating flying performance and durability. The feeling and controllability can be improved in a favorable way. The dimensions and densities of these elements are adjusted as follows. An intermediate layer having a thickness of at least 1 mm, a specific gravity of less than 1.2, and a hardness of at least 85 on JIS C scale is formed around a center core having a diameter of at least 29 mm and a specific gravity of less than 1.4 and greater than the intermediate layer specific gravity and a cover having a thickness of 1 to 3 mm is formed on the outer surface of the intermediate layer to complete a solid golf ball. Then even when the center core is softened to a JIS C scale hardness of 45 to 80 and the cover softened to a JIS C scale hardness of 50 to 85, good feeling and controllability can be achieved with little or no loss of flying distance and durability. Further when the intermediate layer is formed of a resin composition based on a high repulsion ionomer resin, the hitting feel and controllability can be further improved with little or no loss of flying distance and durability. A specific proposal herein is therefore a three-piece solid golf ball according to claim 1. BRIEF DESCRIPTION OF THE DRAWING The only figure, FIG. 1 is a schematic cross section of a three-piece solid golf ball. EXPLANATIONS; PREFERRED AND OPTIONAL FEATURES Referring to FIG. 1, there is schematically illustrated a typical three-piece solid golf ball embodying our new concepts. The ball includes a spherical center core 1 forming the center of the ball and a cover 3 forming the outermost layer of the ball. A relatively hard intermediate layer 2 is disposed between the core 1 and the cover 3. The size and specific gravity of the core 1, intermediate layer 2, and cover 3 are in the specific ranges explained below (which may be selected individually, independently of one another). The center core generally has a diameter of at least 29 mm, preferably 29 to 37 mm and a specific gravity of less than 1.4, preferably 1.05 to 1.38. With a diameter of less than 29 mm, the intermediate layer must be relatively thick, causing loss of repulsion and feeling. With a specific gravity of 1.4 or more, the ball has a heavier weight which may exceed the weight requirement for golf balls. On impact entailing substantial deformation, as found on driver shots, the player gets a feeling which largely depends on the hardness of the center core 1 and varies with the club head speed given by the player. Therefore, the hardness of the center core 1 should be set in accordance with the head speed of the target players. In this sense, the center core hardness ranges from 45 to 80, more preferably from 60 to 80 on JIS C scale (at the center core surface). The center core 1 may be formed from a well-known rubber composition comprising a base rubber, co-crosslinking agent and peroxide through heating, pressing and molding steps. The base rubber may be one conventionally used in solid golf balls and preferably is selected from polybutadiene rubber and mixtures of polybutadiene rubber and polyisoprene rubber. Use of 1,4-polybutadiene rubber containing more than 90% of cis structure is preferred for high repulsion. The co-crosslinking agents used in conventional solid golf balls include zinc and magnesium salts of unsaturated fatty acids such as methacrylic acid and acrylic acid and esters of unsaturated fatty acids such as trimethyl-propane trimethacrylate and they may be used. Zinc acrylate is preferred for high repulsion. The co-crosslinking agent is blended in amounts of about 15 to 30 parts by weight per 100 parts by weight of the base rubber. The peroxide may be selected from a variety of peroxides, preferably dicumyl peroxide and mixtures of dicumyl peroxide and 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane. The peroxide is blended in amounts of about 0.5 to 1 part by weight per 100 parts by weight of the base rubber. If desired, zinc oxide and barium sulfate may be blended in the rubber composition for specific gravity adjustment while antioxidants may also be blended. The intermediate layer 2 has a radial thickness of at least 1 mm, preferably 1.5 to 3.5 mm, a specific gravity of less than 1.2, preferably 0.9 to 1 and lower than the center core specific gravity, and a hardness of at least 85, preferably 85 to 100 on JIS C scale. With a thickness of less than 1 mm, repulsion is lowered to reduce flying distance. With a specific gravity of 1.2 or more, the center core must have a relatively low specific gravity so that the golf ball may be increased in inertia moment and reduced in spin property and thus lose some controllability. Similar detrimental effect is observed when the intermediate layer specific gravity is greater than the center core specific gravity. A layer with a JIS C scale hardness of less than 85 detracts from flying performance. The intermediate layer preferably has an outer diameter of 38 to 41 mm though not limited thereto. Also preferably the difference in specific gravity between the center core and the intermediate layer is 0.1 or more, especially 0.1 to 0.5 though not limited thereto. The intermediate layer 2 is effective in compensating for lower repulsion of the center core 1 which is made soft. It is formed of a relatively hard (JIS C scale hardness ≧ 85), repulsive material. Although the material is not critical, ionomer resins are preferred e.g. having the compositions of Himilan® 1706 or 1605 (commercially available from Mitsui-duPont Polychemical K.K.) or of Surlyn® (commercially available from E.I. du Pont). A 1:1 blend of Himilan 1706 and Himilan 1605 is most preferred. In addition to the ionomer resin, the composition of which the intermediate layer is formed may further contain weight control agents, for example, inorganic fillers such as zinc oxide and barium sulfate, coloring agents such as titanium dioxide, and other additives. The cover 3 generally has a radial thickness of 1 to 3 mm, preferably 1.5 to 2.5 mm. A cover of more than 3 mm thick is low in repulsion whereas a cover of less than 1 mm thick is low in durability such as cut resistance. The hardness of the cover 3 is preferably set in a relatively soft range of 50 to 85, more preferably 60 to 85 on JIS C scale because in this range, good properties in all of repulsion (flying performance), durability and controllability are expected. The cover 3 may be formed of resinous materials which are conventionally used as the cover of solid golf balls, preferably those materials which are relatively soft (JIS C scale hardness 50 to 85) and highly repulsive. Examples include ionomer resins such as Himilan® 1650 commercially available from Mitsui-duPont Polychemical K.K., Surlyn® 8120 commercially available from E.I. duPont, and mixtures thereof, thermoplastic polyester elastomers such as Hytrel® 4047 commercially available from Toray-duPont K.K., and balata resins. If necessary, inorganic fillers may be blended in these resins for coloring purposes. EXAMPLE Examples of the present invention are given below by way of illustration and not by way of limitation. Examples and Comparative Examples Using centre cores, intermediate layers, and covers having the compositions shown in Table 1, three-piece solid golf balls (Examples 1-6, Comparative Examples 1-3) were prepared. The center core was prepared by kneading the respective components in a roll mill and pressure molding at 155°C for 15 minutes. The intermediate layer was formed by injection molding so as to enclose the outer surface of the center core. The cover was formed around the intermediate layer by injection molding. The three-piece solid golf balls were completed in this way. The parameters associated with the core, intermediate layer and cover are shown in Table 2. The golf balls were evaluated for spin characteristic, flying performance, feeling, and durability by the following tests. The results are shown in Table 2. Spin characteristic Using a swing robot manufactured by True Temper Co., the ball was hit by a driver at a head speed of 45 m/s (abbreviated as W1 HS45 in Table 2) and by a sand wedge at a head speed of 17.6 m/s (abbreviated as SW HS17.6 in Table 2). The ball spin (rpm) was observed using a "science eye" (manufactured by Bridgestone Corporation). Feeling Professional golfers evaluated a feeling on impact according to the following criterion. O: good Δ: average ×: poor Flying performance In the spin and feeling tests, the flying distance the ball traveled was also measured. Total evaluation was made according to the following criterion. O: good Δ: average ×: poor Durability Using a flywheel hitting machine, each ball was hit repeatedly at a head speed of 38 m/s until it was broken. With the number of hits counted, the ball was rated according to the following criterion. O : good Example Comparative Example 1 2 3 4 5 6 1 2 3 Center core Cis-1,4-polybutadiene 100 100 100 100 100 100 100 100 100 Zinc acrylate 20 20 20 30 20 20 20 25 20 Zinc oxide 56 36 36 20 23 10 90 25 55 Antioxidant 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Dicumyl peroxide 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 Intermediate layer Himilan 1706 50 50 50 50 50 50 50 50 50 Himilan 1605 50 50 50 50 50 50 50 50 50 Cover Himilan 1650 50 50 50 50 50 50 Surlyn 8120 50 50 50 50 50 50 Hytrel 4047 100 100 Trans-isoprene rubber 90 Natural rubber 10 Note: The amounts of components blended are parts by weight and their proportion is independent among the center core, intermediate layer, and cover. Example Comparative Example 1 2 3 4 5 6 1 2 3 Center core Outer diameter, mm 31.52 35.28 35.28 35.28 35.29 36.40 27.68 35.24 31.52 Hardness, JIS C 66 66 66 79 66 66 66 73 66 Specific gravity 1.36 1.24 1.24 1.19 1.16 1.07 1.56 1.19 1.35 Intermediate layer Thickness, mm 3.4 1.7 2.2 2.2 1.7 2.0 5.7 1.8 1.6 Hardness, JIS C 91 91 91 91 91 91 91 82 91 Specific gravity 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.97 0.95 Outer diameter, mm 38.35 38.73 39.65 39.66 38.73 40.40 39.00 38.91 34.56 Cover Thickness, mm 2.2 2.0 1.5 1.5 2.0 1.8 1.8 1.9 4.0 Specific gravity 0.97 0.97 0.97 1.10 1.13 0.97 0.97 1.10 0.97 Hardness, JIS C 82 82 82 61 78 82 82 61 82 Ball Outer diameter, mm 42.68 42.67 42.67 42.70 42.70 44.00 42.65 42.63 42.65 Weight, g 45.50 45.45 45.50 45.55 45.53 45.60 45.50 45.55 45.50 Performance Spin (rpm) W1 HS45 3300 3020 3030 3920 3600 3030 3500 3600 3250 SW HS17.6 3900 4000 4300 6390 5800 4100 4100 4050 3500 Feeling Δ ○ ○ Δ ○ ○ × ○ ○ Flying performance ○ ○ ○ ○ Δ ○ × × × Durability ○ ○ ○ ○ ○ ○ ○ ○ ○ As is evident from Table 2, the three-piece solid golf balls, and particularly those with the preferred selected dimensions and densities etc. of their components, had a good balance of properties in that the center core and cover can be made soft so as to ensure a pleasant feeling and controllability (spin) without deteriorating flying performance and durability. There has been described a three-piece solid golf ball which includes a core, intermediate layer and cover having controlled size, hardness and specific gravity so that the ball has a good total balance of properties in that a relatively soft center core and cover are used to ensure a pleasant feeling and controllability at no sacrifice of flying performance and durability. Japanese Patent Application No. 5-193065 is incorporated herein by reference. Although some preferred embodiment have been described, many modifications and variations may be made thereto in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims: A three-piece solid golf ball comprising a centre core (1), an intermediate layer (2) around the core and a cover (3) enclosing the core (1) and intermediate layer (2), the centre core (1) having a diameter of at least 29 mm, a specific gravity less than 1.4 and a hardness in the range from 45 to 80 on the JIS-C scale; the intermediate layer having a thickness of at least 1 mm and a specific gravity which is less than 1.2 and less than the specific gravity of the centre core (1); the cover (3) having a thickness in the range from 1 to 3 mm; and wherein the intermediate layer (2) has a hardness of at least 85 on the JIS C scale and the cover (3) is softer than the intermediate layer. A golf ball according to claim 1 in which the intermediate layer (2) is made of a composition based on a high-repulsion ionomer resin. A golf ball according to claim 1 or claim 2 in which the cover (3) has a hardness in the range from 50 to 85 on the JIS C scale. A golf ball according to any one of the preceding claims in which the intermediate layer (2) has a thickness in the range from 1.5 to 3.5 mm. A golf ball according to any one of the preceding claims in which the intermediate layer (2) has a specific gravity in the range from 0.9 to 1, and at least 0.1 less than the specific gravity of the core (1). A golf ball according to any one of the preceding claims in which the intermediate layer has an outer diameter in the range from 38 to 41 mm. A golf ball according to any one of the preceding claims in which the centre core (1) is made of a composition based on polybutadiene rubber.

Inventors: EGASHIRA YOSHINORI, HIGUCHI HIROSHI, YAMADA TADATOSHI, YAMAGISHI HISASHI, EGASHIRA, YOSHINORI, HIGUCHI, HIROSHI, YAMADA, TADATOSHI, YAMAGISHI, HISASHI
Applicants: BRIDGESTONE SPORTS CO LTD, BRIDGESTONE SPORTS CO., LTD.
Application year: 1994
Main classification: A63B37
IPC codes: A63B37